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- Author or Editor: William D. Scott x
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Abstract
A stochastic numerical cloud model is used to investigate simultaneously growth of precipitation, the formation of electrical charges on the particles, and the development of the ambient electric field utilizing the polarization charging mechanism. The results indicate a close coupling between precipitation growth and electrification. Precipitation is reduced when the electric field reaches magnitudes of kilovolts per centimeter. The distributions of charge on the particles show charges of a realistic magnitude. Simple restraints on the coalescence efficiency based on electric charge show that, indeed particle charges can have a profound effect on rain development through coalescence. The overall results qualitatively agree with the results from the continuous collection model of Ziv and Levin, i.e., the partial levitation of the particles due to electrical forces and the termination of electric field growth can occur at electric field strengths large enough for lightning.
Abstract
A stochastic numerical cloud model is used to investigate simultaneously growth of precipitation, the formation of electrical charges on the particles, and the development of the ambient electric field utilizing the polarization charging mechanism. The results indicate a close coupling between precipitation growth and electrification. Precipitation is reduced when the electric field reaches magnitudes of kilovolts per centimeter. The distributions of charge on the particles show charges of a realistic magnitude. Simple restraints on the coalescence efficiency based on electric charge show that, indeed particle charges can have a profound effect on rain development through coalescence. The overall results qualitatively agree with the results from the continuous collection model of Ziv and Levin, i.e., the partial levitation of the particles due to electrical forces and the termination of electric field growth can occur at electric field strengths large enough for lightning.
Abstract
The formation of multiple layers of adsorbed water molecules on the basal and prism faces of ice may be responsible for the remarkable temperature dependence of all growth variables (linear growth rate, step velocity, and mean migration distance). This effect results from an increased residence time of molecules in the adsorbed state as the melting point is approached. A quantitative treatment based on the Brunauer, Emmett and Teller model of multi-layer adsorption exemplifies these concepts and appears to explain the measured trends with temperature. When the theoretical treatment is used in conjunction with a growth model based on the propagation of spiral steps, reasonable values for the condensation coefficient emerge. The alternation of the primary habit of ice crystals with temperature is explained when the theoretical treatment is applied to the basal and prism faces, respectively.
Abstract
The formation of multiple layers of adsorbed water molecules on the basal and prism faces of ice may be responsible for the remarkable temperature dependence of all growth variables (linear growth rate, step velocity, and mean migration distance). This effect results from an increased residence time of molecules in the adsorbed state as the melting point is approached. A quantitative treatment based on the Brunauer, Emmett and Teller model of multi-layer adsorption exemplifies these concepts and appears to explain the measured trends with temperature. When the theoretical treatment is used in conjunction with a growth model based on the propagation of spiral steps, reasonable values for the condensation coefficient emerge. The alternation of the primary habit of ice crystals with temperature is explained when the theoretical treatment is applied to the basal and prism faces, respectively.
Abstract
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Abstract
High quality near-bottom boundary layer measurements obtained at a midshelf location (90 m water depth) in the CODE region off Northern California are described. Bottom tripod velocity measurements and supporting data obtained during typical spring and early summer conditions (June 1981 during CODE-1) are analyzed to obtain bath velocity profiles and mean bottom stress and bottom roughness estimates. During the time period described, the mean near-bottom (<2 m) velocity profile are highly logarithmic (R>0.997) approximately 30 percent of the time. Effects induced by unsteadiness from internal waves result in some degradation of the profiles (0.96≤R≤0.997) the rest of the time. Mean stress profiles indicate the logarithmic layer is approximately a constant-stress layer. The near-bottom flow field is Composed of mean currents and oscillatory currents due to well. Typing mean u * values estimated from measurements greater than 30 cm above the bottom have magnitudes of 0.5–1.0 cm s−1. Mean stress values are three to seven times larger than expected from predictions using a typical smooth-bottom drag coefficient and one-and-one-half to three-and-one-half times larger than expected for predictions using a drag coefficient based on the observed rough bottom. Corresponding z 0 values have magnitude of approximately 1 cm, an order of magnitude larger than the observed physical bottom roughness. These values are demonstrated to he consistent with those expected from theoretical models for combined wave and current flows. The u * values estimated from the CODE-1 data and predicted by the Grant and Madsen model typically agree within 10–15 percent.
The waves influencing the midshelf bottom-stress estimates are 12–20 second swell associated with distant Pacific storms. Them waves are present over most of the year. The results demonstrate that waves must be taken into account in predicting bottom stress over the Northern California Shelf and that these predictions can be made using existing theory.
Abstract
High quality near-bottom boundary layer measurements obtained at a midshelf location (90 m water depth) in the CODE region off Northern California are described. Bottom tripod velocity measurements and supporting data obtained during typical spring and early summer conditions (June 1981 during CODE-1) are analyzed to obtain bath velocity profiles and mean bottom stress and bottom roughness estimates. During the time period described, the mean near-bottom (<2 m) velocity profile are highly logarithmic (R>0.997) approximately 30 percent of the time. Effects induced by unsteadiness from internal waves result in some degradation of the profiles (0.96≤R≤0.997) the rest of the time. Mean stress profiles indicate the logarithmic layer is approximately a constant-stress layer. The near-bottom flow field is Composed of mean currents and oscillatory currents due to well. Typing mean u * values estimated from measurements greater than 30 cm above the bottom have magnitudes of 0.5–1.0 cm s−1. Mean stress values are three to seven times larger than expected from predictions using a typical smooth-bottom drag coefficient and one-and-one-half to three-and-one-half times larger than expected for predictions using a drag coefficient based on the observed rough bottom. Corresponding z 0 values have magnitude of approximately 1 cm, an order of magnitude larger than the observed physical bottom roughness. These values are demonstrated to he consistent with those expected from theoretical models for combined wave and current flows. The u * values estimated from the CODE-1 data and predicted by the Grant and Madsen model typically agree within 10–15 percent.
The waves influencing the midshelf bottom-stress estimates are 12–20 second swell associated with distant Pacific storms. Them waves are present over most of the year. The results demonstrate that waves must be taken into account in predicting bottom stress over the Northern California Shelf and that these predictions can be made using existing theory.
Abstract
Observations are presented in which the standard dual-polarization meteorological quantities (Z DR, ϕ dp , and ρ HV ) are determined from simultaneous horizontal (H) and vertical (V) transmissions. The return signals are measured in parallel H and V receiving channels. Because the parameters are determined from simultaneous measurements they are not affected by Doppler phase shifts that increase the variance of ϕ dp and ρ HV when alternating H and V polarizations are transmitted. The approach has the additional advantage that a high-power polarization switch is not needed. The relative phases of the H and V components were such that the transmitted polarization was circular. Circular polarization is shown to detect horizontally oriented particles such as rain with the same effectiveness as linearly polarized transmissions, and optimally detects randomly oriented or shaped particles such as hail. Circular polarization also optimally senses nonhorizontally oriented particles such as electrically aligned ice crystals. By not needing to alternate between H and V transmissions it becomes practical to make polarization-diverse measurements by transmitting other orthogonal polarizations on successive pulses (e.g., left-hand circular and +45° slant linear) to aid in identifying precipitation types. It is shown that ρ HV from simultaneous transmissions provides the same information on randomly oriented scatterers as the linear depolarization ratio LDR from H or V transmissions, and that LDR does not need to be measured when information on particle canting is not important or is not needed.
Abstract
Observations are presented in which the standard dual-polarization meteorological quantities (Z DR, ϕ dp , and ρ HV ) are determined from simultaneous horizontal (H) and vertical (V) transmissions. The return signals are measured in parallel H and V receiving channels. Because the parameters are determined from simultaneous measurements they are not affected by Doppler phase shifts that increase the variance of ϕ dp and ρ HV when alternating H and V polarizations are transmitted. The approach has the additional advantage that a high-power polarization switch is not needed. The relative phases of the H and V components were such that the transmitted polarization was circular. Circular polarization is shown to detect horizontally oriented particles such as rain with the same effectiveness as linearly polarized transmissions, and optimally detects randomly oriented or shaped particles such as hail. Circular polarization also optimally senses nonhorizontally oriented particles such as electrically aligned ice crystals. By not needing to alternate between H and V transmissions it becomes practical to make polarization-diverse measurements by transmitting other orthogonal polarizations on successive pulses (e.g., left-hand circular and +45° slant linear) to aid in identifying precipitation types. It is shown that ρ HV from simultaneous transmissions provides the same information on randomly oriented scatterers as the linear depolarization ratio LDR from H or V transmissions, and that LDR does not need to be measured when information on particle canting is not important or is not needed.
Abstract
This paper presents measured data related to the question of how constant are “constant-level” balloons. The simultaneous use of two balloon-borne instruments, a radio altimeter and a pressure sensor, operating on entirely different principles, help to distinguish between sensor noise and true balloon altitude fluctuation. Four types of superpressure balloon altitude changes at the level of 150 mb were observed: (i) neutral buoyancy oscillations (NBO) with a period of about 200 sec and with peak-to-peak amplitude of up to 50 m, (ii) short-term oscillations with a period of ∼1.2 hr and peak-to-peak amplitudes of up to 80 m, (iii) diurnal half-cycle (day observations only) with an amplitude of up to 150 m, and (iv) possible trends of up to 120 m per day.
The data were obtained during four superpressure-balloon 150-mb flights in the Southern Hemisphere. These balloon flights were part of a test program for the TWERL Experiment. NCAR's GHOST balloons and navigation system were used, with the final version of the TWERLE radio altimeter and an early version of the pressure sensor.
The data are presented with a discussion of their limitations, mainly aliasing, ambiguity, and the absolute accuracy of the pressure sensor. A theoretical analysis of the NBO, with a spectrum analysis of supporting ground radar data, are given in the Appendix.
Abstract
This paper presents measured data related to the question of how constant are “constant-level” balloons. The simultaneous use of two balloon-borne instruments, a radio altimeter and a pressure sensor, operating on entirely different principles, help to distinguish between sensor noise and true balloon altitude fluctuation. Four types of superpressure balloon altitude changes at the level of 150 mb were observed: (i) neutral buoyancy oscillations (NBO) with a period of about 200 sec and with peak-to-peak amplitude of up to 50 m, (ii) short-term oscillations with a period of ∼1.2 hr and peak-to-peak amplitudes of up to 80 m, (iii) diurnal half-cycle (day observations only) with an amplitude of up to 150 m, and (iv) possible trends of up to 120 m per day.
The data were obtained during four superpressure-balloon 150-mb flights in the Southern Hemisphere. These balloon flights were part of a test program for the TWERL Experiment. NCAR's GHOST balloons and navigation system were used, with the final version of the TWERLE radio altimeter and an early version of the pressure sensor.
The data are presented with a discussion of their limitations, mainly aliasing, ambiguity, and the absolute accuracy of the pressure sensor. A theoretical analysis of the NBO, with a spectrum analysis of supporting ground radar data, are given in the Appendix.
Abstract
This work addresses the long-term relative occurrence of cloud-to-ground (CG) and intracloud (IC; no attachment to ground) flashes for the contiguous United States (CONUS). It expands upon an earlier analysis by Boccippio et al. who employed 4-yr datasets provided by the U.S. National Lightning Detection Network (NLDN) and the Optical Transient Detector (OTD). Today, the duration of the NLDN historical dataset has more than tripled, and OTD data can be supplemented with data from the Lightning Imaging Sensor (LIS). This work is timely, given the launch of GOES-16, which includes the world’s first geostationary lightning mapper that will observe total lightning (IC and CG) over the Americas and adjacent ocean regions. Findings support earlier results indicating factor-of-10 variations in the IC:CG ratio throughout CONUS, with climatological IC fraction varying between 0.3 and greater than 0.9. The largest values are seen in the Pacific Northwest, central California, and where Colorado borders Kansas and Nebraska. An uncertainty analysis indicates that the large values in the northwest and central California are likely not due to measurement uncertainty. The high IC:CG ratio (>4) throughout much of Texas reported by Boccippio et al. is not supported by this longer-term climatology. There is no clear evidence of differences in IC fraction between land and coastal ocean. Lightning characteristics in six selected large regions show a consistent positive relationship between IC fraction and the percent of positive CG flashes, irrespective of lightning incidence (flash density), dominant season, or diurnal maximum period.
Abstract
This work addresses the long-term relative occurrence of cloud-to-ground (CG) and intracloud (IC; no attachment to ground) flashes for the contiguous United States (CONUS). It expands upon an earlier analysis by Boccippio et al. who employed 4-yr datasets provided by the U.S. National Lightning Detection Network (NLDN) and the Optical Transient Detector (OTD). Today, the duration of the NLDN historical dataset has more than tripled, and OTD data can be supplemented with data from the Lightning Imaging Sensor (LIS). This work is timely, given the launch of GOES-16, which includes the world’s first geostationary lightning mapper that will observe total lightning (IC and CG) over the Americas and adjacent ocean regions. Findings support earlier results indicating factor-of-10 variations in the IC:CG ratio throughout CONUS, with climatological IC fraction varying between 0.3 and greater than 0.9. The largest values are seen in the Pacific Northwest, central California, and where Colorado borders Kansas and Nebraska. An uncertainty analysis indicates that the large values in the northwest and central California are likely not due to measurement uncertainty. The high IC:CG ratio (>4) throughout much of Texas reported by Boccippio et al. is not supported by this longer-term climatology. There is no clear evidence of differences in IC fraction between land and coastal ocean. Lightning characteristics in six selected large regions show a consistent positive relationship between IC fraction and the percent of positive CG flashes, irrespective of lightning incidence (flash density), dominant season, or diurnal maximum period.
Abstract
The Airborne Cloud–Aerosol Transport System (ACATS) is a Doppler wind lidar system that has recently been developed for atmospheric science capabilities at the NASA Goddard Space Flight Center (GSFC). ACATS is also a high-spectral-resolution lidar (HSRL), uniquely capable of directly resolving backscatter and extinction properties of a particle from a high-altitude aircraft. Thus, ACATS simultaneously measures optical properties and motion of cloud and aerosol layers. ACATS has flown on the NASA ER-2 during test flights over California in June 2012 and science flights during the Wallops Airborne Vegetation Experiment (WAVE) in September 2012. This paper provides an overview of the ACATS method and instrument design, describes the ACATS HSRL retrieval algorithms for cloud and aerosol properties, and demonstrates the data products that will be derived from the ACATS data using initial results from the WAVE project. The HSRL retrieval algorithms developed for ACATS have direct application to future spaceborne missions, such as the Cloud–Aerosol Transport System (CATS) to be installed on the International Space Station (ISS). Furthermore, the direct extinction and particle wind velocity retrieved from the ACATS data can be used for science applications such as dust or smoke transport and convective outflow in anvil cirrus clouds.
Abstract
The Airborne Cloud–Aerosol Transport System (ACATS) is a Doppler wind lidar system that has recently been developed for atmospheric science capabilities at the NASA Goddard Space Flight Center (GSFC). ACATS is also a high-spectral-resolution lidar (HSRL), uniquely capable of directly resolving backscatter and extinction properties of a particle from a high-altitude aircraft. Thus, ACATS simultaneously measures optical properties and motion of cloud and aerosol layers. ACATS has flown on the NASA ER-2 during test flights over California in June 2012 and science flights during the Wallops Airborne Vegetation Experiment (WAVE) in September 2012. This paper provides an overview of the ACATS method and instrument design, describes the ACATS HSRL retrieval algorithms for cloud and aerosol properties, and demonstrates the data products that will be derived from the ACATS data using initial results from the WAVE project. The HSRL retrieval algorithms developed for ACATS have direct application to future spaceborne missions, such as the Cloud–Aerosol Transport System (CATS) to be installed on the International Space Station (ISS). Furthermore, the direct extinction and particle wind velocity retrieved from the ACATS data can be used for science applications such as dust or smoke transport and convective outflow in anvil cirrus clouds.
Abstract
A revised Bayesian algorithm for estimating surface rain rate, convective rain proportion, and latent heating profiles from satellite-borne passive microwave radiometer observations over ocean backgrounds is described. The algorithm searches a large database of cloud-radiative model simulations to find cloud profiles that are radiatively consistent with a given set of microwave radiance measurements. The properties of these radiatively consistent profiles are then composited to obtain best estimates of the observed properties. The revised algorithm is supported by an expanded and more physically consistent database of cloud-radiative model simulations. The algorithm also features a better quantification of the convective and nonconvective contributions to total rainfall, a new geographic database, and an improved representation of background radiances in rain-free regions. Bias and random error estimates are derived from applications of the algorithm to synthetic radiance data, based upon a subset of cloud-resolving model simulations, and from the Bayesian formulation itself. Synthetic rain-rate and latent heating estimates exhibit a trend of high (low) bias for low (high) retrieved values. The Bayesian estimates of random error are propagated to represent errors at coarser time and space resolutions, based upon applications of the algorithm to TRMM Microwave Imager (TMI) data. Errors in TMI instantaneous rain-rate estimates at 0.5°-resolution range from approximately 50% at 1 mm h−1 to 20% at 14 mm h−1. Errors in collocated spaceborne radar rain-rate estimates are roughly 50%–80% of the TMI errors at this resolution. The estimated algorithm random error in TMI rain rates at monthly, 2.5° resolution is relatively small (less than 6% at 5 mm day−1) in comparison with the random error resulting from infrequent satellite temporal sampling (8%–35% at the same rain rate). Percentage errors resulting from sampling decrease with increasing rain rate, and sampling errors in latent heating rates follow the same trend. Averaging over 3 months reduces sampling errors in rain rates to 6%–15% at 5 mm day−1, with proportionate reductions in latent heating sampling errors.
Abstract
A revised Bayesian algorithm for estimating surface rain rate, convective rain proportion, and latent heating profiles from satellite-borne passive microwave radiometer observations over ocean backgrounds is described. The algorithm searches a large database of cloud-radiative model simulations to find cloud profiles that are radiatively consistent with a given set of microwave radiance measurements. The properties of these radiatively consistent profiles are then composited to obtain best estimates of the observed properties. The revised algorithm is supported by an expanded and more physically consistent database of cloud-radiative model simulations. The algorithm also features a better quantification of the convective and nonconvective contributions to total rainfall, a new geographic database, and an improved representation of background radiances in rain-free regions. Bias and random error estimates are derived from applications of the algorithm to synthetic radiance data, based upon a subset of cloud-resolving model simulations, and from the Bayesian formulation itself. Synthetic rain-rate and latent heating estimates exhibit a trend of high (low) bias for low (high) retrieved values. The Bayesian estimates of random error are propagated to represent errors at coarser time and space resolutions, based upon applications of the algorithm to TRMM Microwave Imager (TMI) data. Errors in TMI instantaneous rain-rate estimates at 0.5°-resolution range from approximately 50% at 1 mm h−1 to 20% at 14 mm h−1. Errors in collocated spaceborne radar rain-rate estimates are roughly 50%–80% of the TMI errors at this resolution. The estimated algorithm random error in TMI rain rates at monthly, 2.5° resolution is relatively small (less than 6% at 5 mm day−1) in comparison with the random error resulting from infrequent satellite temporal sampling (8%–35% at the same rain rate). Percentage errors resulting from sampling decrease with increasing rain rate, and sampling errors in latent heating rates follow the same trend. Averaging over 3 months reduces sampling errors in rain rates to 6%–15% at 5 mm day−1, with proportionate reductions in latent heating sampling errors.
Abstract
The Community Climate System Model version 3 (CCSM3) has recently been developed and released to the climate community. CCSM3 is a coupled climate model with components representing the atmosphere, ocean, sea ice, and land surface connected by a flux coupler. CCSM3 is designed to produce realistic simulations over a wide range of spatial resolutions, enabling inexpensive simulations lasting several millennia or detailed studies of continental-scale dynamics, variability, and climate change. This paper will show results from the configuration used for climate-change simulations with a T85 grid for the atmosphere and land and a grid with approximately 1° resolution for the ocean and sea ice. The new system incorporates several significant improvements in the physical parameterizations. The enhancements in the model physics are designed to reduce or eliminate several systematic biases in the mean climate produced by previous editions of CCSM. These include new treatments of cloud processes, aerosol radiative forcing, land–atmosphere fluxes, ocean mixed layer processes, and sea ice dynamics. There are significant improvements in the sea ice thickness, polar radiation budgets, tropical sea surface temperatures, and cloud radiative effects. CCSM3 can produce stable climate simulations of millennial duration without ad hoc adjustments to the fluxes exchanged among the component models. Nonetheless, there are still systematic biases in the ocean–atmosphere fluxes in coastal regions west of continents, the spectrum of ENSO variability, the spatial distribution of precipitation in the tropical oceans, and continental precipitation and surface air temperatures. Work is under way to extend CCSM to a more accurate and comprehensive model of the earth's climate system.
Abstract
The Community Climate System Model version 3 (CCSM3) has recently been developed and released to the climate community. CCSM3 is a coupled climate model with components representing the atmosphere, ocean, sea ice, and land surface connected by a flux coupler. CCSM3 is designed to produce realistic simulations over a wide range of spatial resolutions, enabling inexpensive simulations lasting several millennia or detailed studies of continental-scale dynamics, variability, and climate change. This paper will show results from the configuration used for climate-change simulations with a T85 grid for the atmosphere and land and a grid with approximately 1° resolution for the ocean and sea ice. The new system incorporates several significant improvements in the physical parameterizations. The enhancements in the model physics are designed to reduce or eliminate several systematic biases in the mean climate produced by previous editions of CCSM. These include new treatments of cloud processes, aerosol radiative forcing, land–atmosphere fluxes, ocean mixed layer processes, and sea ice dynamics. There are significant improvements in the sea ice thickness, polar radiation budgets, tropical sea surface temperatures, and cloud radiative effects. CCSM3 can produce stable climate simulations of millennial duration without ad hoc adjustments to the fluxes exchanged among the component models. Nonetheless, there are still systematic biases in the ocean–atmosphere fluxes in coastal regions west of continents, the spectrum of ENSO variability, the spatial distribution of precipitation in the tropical oceans, and continental precipitation and surface air temperatures. Work is under way to extend CCSM to a more accurate and comprehensive model of the earth's climate system.
